Fixed bug segfault due to EX(opline) might be NULL

this bug only exists in PHP-5.5

reproduce script:

````
<?php

xhprof_enable(XHPROF_FLAGS_CPU | XHPROF_FLAGS_MEMORY);
function load($name) {
}
spl_autoload_register("load");

new A();
````

will segfault with backtrace:

````
Program received signal SIGSEGV, Segmentation fault.
0x00002aaaaef62494 in hp_execute_internal (execute_data=0x7fffffffa8c0, fci=0x7fffffffaa70, ret=1)
    at /home/huixinchen/opensource/github/xhprof/extension/xhprof.c:1708
1708	    temp_variable *retvar = &EX_T(opline->result.var);
(gdb) bt
#0  0x00002aaaaef62494 in hp_execute_internal (execute_data=0x7fffffffa8c0, fci=0x7fffffffaa70, ret=1)
    at /home/huixinchen/opensource/github/xhprof/extension/xhprof.c:1708
#1  0x00000000008d9d71 in zend_call_function (fci=0x7fffffffaa70, fci_cache=0x7fffffffaa40)
    at /home/huixinchen/opensource/php-5.5/Zend/zend_execute_API.c:959
#2  0x00000000008da805 in zend_lookup_class_ex (name=0x2aaaaf26f718 "A", name_length=1, key=0x2aaaaf69b800,
    use_autoload=1, ce=0x7fffffffabb0) at /home/huixinchen/opensource/php-5.5/Zend/zend_execute_API.c:1115
#3  0x00000000008db90a in zend_fetch_class_by_name (class_name=0x2aaaaf26f718 "A", class_name_len=1,
    key=0x2aaaaf69b800, fetch_type=4) at /home/huixinchen/opensource/php-5.5/Zend/zend_execute_API.c:1595
#4  0x000000000092d15c in ZEND_FETCH_CLASS_SPEC_CONST_HANDLER (execute_data=0x2aaaab156128)
    at /home/huixinchen/opensource/php-5.5/Zend/zend_vm_execute.h:1197
#5  0x000000000092a12b in execute_ex (execute_data=0x2aaaab156128)
    at /home/huixinchen/opensource/php-5.5/Zend/zend_vm_execute.h:363
#6  0x000000000092a1b4 in zend_execute (op_array=0x2aaaab191650)
    at /home/huixinchen/opensource/php-5.5/Zend/zend_vm_execute.h:388
````